I love these hair ties and use them all the time on my pony tail. I love how easy they are to put on and take off. I used to go through them pretty quickly, as they wear out after a week or two of use.
Then I bought some elastic at a fabric store, replaced the elastic, and one ball hair tie lasted me 6 months on one piece of elastic.
1 worn-out ball hair tie
1 pair of scissors (or something else to cut with)
elastic from a fabric store, online store, etc.
1 ball hair tie in good condition that fits you well (optional)
Step 1: Measuring
Cut the old elastic off the worn-out hair tie.
If you don't want to compare the length with a good hair tie, you can just cut a length of 21 cm (8.25 in) of the elastic.
If you do want to compare, measure the good hair tie's length when at rest and when stretched out. Thread some elastic through the two balls that you already cut the old elastic off of. Hold the ends and measure the length at rest and extended. Adjust the amount of elastic until you have a good match. Hold the attached end of the elastic (where you'd cut it to keep it that length), remove the balls, and measure it. Add about 1cm or 3/8ths of an inch to leave room for the knot. Make a note of the measurement for future use and cut a piece of elastic that length.
Step 2: The Knot
Tie the ends of the elastic together in an offset water knot, as shown. As you tighten the knot, try to get it close to the end. Once tight, trim the ends, leaving a little room so it doesn't come undone.
Step 3: Complete
You're done! I find it works fine without any clamp in the middle.
If it's too loose or too tight, try starting over and adjusting the length of the elastic.